The are many instances when it is desirable or necessary to control the amount of light passing through lens or lens structure. One of these instances relates to the lenses used in eyewear. Continuous control of optical transmissivity across a broad range of magnitudes is particularly desirable in many medical applications such as diagnosis and treatment of retinal disease, visual field abnormalities known a scotomas, optic neuropathy, macular degeneration, and the like. Diagnosis of these conditions can be problematic using uncontrolled ambient light since the magnitude of the symptoms of these diseases can vary with ambient light levels.
For patients suffering from retinal diseases, sudden changes in ambient light levels, such as emerging from a dimly lit room into a bright sunny day, and vise versa, can cause serious problems and momentary blindness. It would be desirable to maintain these patients in a partially dark adapted state. This partially dark adapted state would involve maintaining a constant light level at the patients eyes despite variations in ambient light levels.
Existing devices for controlling optical transmissivity are not suited for the applications described. Many of these devices cannot continuously control transmissivity over a broad range of ambient light levels. An example of this type of device is a flash blindness device that darkens only when a sudden change in ambient light levels is detected. One such device is disclosed in U.S. Pat. No. 3,245,315 to Marks et al.
Other devices that control optical transmissivity have their transmissivity dependent on ambient light levels. These devices locate their sensors to detect the ambient light levels and not the transmitted light levels. Transmitted light through these devices is not constant and is dependant on the ambient light levels. Ideally, for the applications described, transmitted light should be independent of ambient light levels and constant within a narrow range. Examples of devices with transmitted light levels dependant on ambient light levels are U.S. Pat. No. 5,015,086 to Okaue et al. and U.S. Pat. No. 4,968,127 to Russell et al.
What is needed is a device to continuously control optical transmissivity over a broad range wherein the transmitted light is independent of ambient light levels.